1. Field of the Invention
This invention relates to a semiconductor device which is able to accomplish high speed switchings with a large current. In particular, this invention relates to a semiconductor device whose internal inductance is greatly reduced so as to reduce its surge voltage.
2. Description of the Prior Art
Generally speaking, a semiconductor device for use with a large current has the basic structure, which is described by means of the equivalent circuit shown in FIG. 10, comprised of two transistors and two diodes. This devices is used in the field of power electronics to accomplish power conversion, power control and so on.
Such a device is mainly applied to a three-phase motor driving circuit. In FIG. 11, the structure of this circuit is shown as an example. As shown in this figure, six control transistors are needed for driving the three-phase motor. Thus, three semiconductor devices having a two circuit structure shown in FIG. 10 are necessary in order to construct the driving circuit.
Besides the circuit shown in FIG. 10, the semiconductor device for use with a large current may have the circuit structure shown in FIG. 12a or 12b. The circuit shown in FIG. 12a has a structure comprised of one circuit and the other shown in FIG. 12b has a structure comprised of two circuits.
The motor control circuit, in which the above mentioned semiconductor device is applied, is usually driven in the PWM (Pulse Width Modulation) control mode. The carrier frequency of this mode has become over 10 KHz through the development of semiconductor devices. As the carrier frequency goes high, the magnitude of a noise generated by a motor becomes small. In order to increase the carrier frequency, the semiconductor device should be switched at a very high speed. In this case, however, a voltage is induced into inductance L in a main circuit according to the abrupt current change arisen in this circuit at every switching. As a result, a surge voltage -L (di/dt) is generated. When the magnitude of this surge voltage is extremely large, semiconductor elements contained in the semiconductor device may be destroyed. In addition, this surge voltage becomes a cause of wrong operations and the destruction of external control and protection circuits. Consequently, the surge voltage should be reduced as much as possible.
The magnitude of a surge voltage largely depends on inductance L. In order to reduce the magnitude of this surge voltage, therefore, external wirings should be so arranged that the magnitude of inductance L becomes minimum. Also, as shown in FIGS. 13a and 13b, a snubber circuit, made of a resister, a condenser, at least one diode and others, is provided to reduce the magnitude of inductance L.
In the above mentioned prior art, however, countermeasures only for the external inductance L of the semiconductor device have been taken. In other words, no countermeasure is taken for the reduction of internal inductance L arising on the internal circuit of the semiconductor device.
Thus, the reduction of the magnitude of inductance L arising on the internal circuit is highly desirable in order to lower the surge voltage as far as possible.
As shown in FIG. 10, the internal inductance L of the semiconductor device is composed of the following: inductance L.sub.C1 at the first power terminal; inductance L.sub.C2E1 at the second power terminal; inductance L.sub.E2 at the third power terminal; and inductance LB'g at the bonding wire part.
In general, the self-inductance of a conductor, which is a cylinder having a length l, a radius a, and a permeability .mu. as shown in FIG. 14a, is expressed as follows as far as a current flows uniformly through the sectional area of the conductor: ##EQU1##
If there are two cylinders, one of which has a length l and a radius a and the other of which has a length l and a radius b, arranged parallel having distance d as shown in FIG. 14b, mutual inductance M arises as follows, in addition to said self-inductance L.sub.s : ##EQU2## In this case, the mutual inductance M has a positive value when both currents on the respective cylinders flow in the same direction. On the contrary, when each current on the respective cylinders flows in opposite directions, M has a negative value.
Then, the total inductance L of the conductors shown in FIG. 14b is written as follows: EQU L=Ls+M (3)
As is evident from the above formulas, the inductance L strongly depends on the length l of the conductors, their sectional areas, distance d between them, and the directions of the currents.
In the prior art semiconductor device, the internal wirings for power terminals are long in length and small in sectional area, and thus, each power terminal has a large self-inductance value Ls.
In addition, the lengths and the current directions among said power terminals can't be set so as to reduce the magnitude of mutual inductance M. As a result, the magnitude of mutual inductance M among the power terminals has become a large value in the prior art device. Accordingly, inductance L.sub.C1, L.sub.C2E1 or L.sub.E2 of respective power terminals shown in FIG. 10 has become a large value in each power terminal.
Still in addition, the current directions on an internal bonding wire and on a substrate can't be set in the prior art device so as to reduce the magnitude of mutual inductance M. Therefore, the magnitude of mutual inductance M in the bonding wires remains large in the prior art device.
As explained above, the internal inductance of the prior art device has a relatively large value with respect to the inductance value arising from external wirings. This fact prevents the reduction of the above mentioned surge voltage.